1. Field of the Invention
The present invention relates to a repeater for a mobile communication system, etc. and more particularly relates to a wireless repeating method and repeater for a Time Division Multiple Access-Time Division Duplex (hereinafter referred to as "TDMA-TDD") communication method communication system.
2. Description of Related Art
FIG. 2(a) and FIG. 2(b) are system configuration drawings for describing a related wireless communication method. Related systems can principally be divided into two methods of a direct repeating method of FIG. 2(a) and a heterodyne transmission method of FIG. 2(b).
In the direct repeating method of FIG. 2(a), electromagnetic waves transmitted from a transmission station a at a frequency fa are received and amplified at a repeater 10 before being transmitted to a receiving station b at the same frequency fa.
The repeater 10 comprises an antenna 1 for receiving wireless signals of a frequency fa transmitted from the transmission station a, a receiver 2 for amplifying signals received at the antenna 1 to a prescribed level, a transmitter 3 for further amplifying signals amplified to the prescribed level at the receiver 2 to a fixed transmission level and an antenna 4 for transmitting signals amplified to the transmission level at this transmitter 3 to the receiving station b.
This direct repeating method uses frequencies in an effective manner while the configuration of the repeater 10 is simple. However, part of the wireless signal of frequency fa transmitted from the antenna 4 is also received at the antenna 1 and wraparound problems due to oscillation phenomenon therefore occur due to these signals being of the same frequency.
Microwaves that have much greater directivity were therefore used in this direct repeating method so as to limit this method to fixed microwave communication where transmission station a and transmission station b were installed in fixed positions, but this placed severe limitations on the position of installation of the repeater 10 in order to avoid wraparound.
On the other hand, in the heterodyne repeating method of FIG. 2(b), electromagnetic waves transmitted at a frequency fa from the transmission station a are received and amplified at the repeater 10a before being converted to a different frequency fb and transmitted to the receiving station b. In addition to the antenna 1, receiver 2, transmitter 3 and antenna 4 that are the same as for FIG. 2(a), the repeater 10A is also provided with a frequency converter 5 between the receiver 2 and the transmitter 3 for converting the wireless signal of frequency fa received and amplified by the receiver to a wireless signal of a different frequency fb. With this configuration, the wireless signal of frequency fa transmitted from the transmission station a is frequency converted at the repeater 10A and relayed to the receiving station b at a wireless signal of frequency fb.
The heterodyne repeating method does not have wraparound problems and is therefore not limited to microwave communications and fixed wireless communications, with the range of uses therefore being broader. It is, however, necessary for the frequencies used to be twice the frequency of the frequencies used in direct repeating methods.
In recent years, mobile communication systems such as portable telephones have rapidly become widespread. As portable telephones are carried by people for personal use, the total number of telephones is extremely large but the frequency of calls made on each telephone has been low. Further, as communication is carried out while moving with portable telephones, it has not been possible to give directivity and it has also been necessary to use frequencies that provide few obstacles to electromagnetic waves due to obstacles in the propagation path. Because of this, frequencies that are used are in reality limited to, for example, the UHF band etc., with these frequencies being utilized in an effective manner.
In this simple portable telephone system where the frequency of calls per one telephone is low, a TDMA-TDD communication method is used where the same frequency is used for transmitting and receiving because the efficiency of frequency utilization is high, and the transmitting and receiving time is divided so that bidirectional communication is carried out. This TDMA-TDD communication method is used in communication between base stations and mobile stations and it has been necessary to increase the number of base stations provided in order to broaden the area in which communication is possible. However, increasing the number of base stations increases the scale of the communication system as well as increasing the cost.
The method where the repeaters 10 and 10A described above are provided exists as a method of broadening the possible call area at a low cost but with the direct repeating method, wraparound problems occur and application of this method to mobile communications is not possible as is. On the other hand, as the heterodyne repeating method requires twice the frequency, this goes against the original purpose of simple portable telephone systems, etc. There can therefore be obtained a repeater capable of repeating in an appropriate manner in the TDMA-TDD communication method.